Micro-optics components are broadly used in devices for optical sensing, communications, fiber optics, integrated optics, silicon photonics, optic al interconnects, optical signal processing, etc. A micro-lens (ML) is an example of micro-optics component utilized in some pixelated imaging systems. In displays, the ML collimates light radiated by an individual pixel source. In detector focal plane arrays (FPA), micro-lenses can be positioned in front of individual pixels to collect light at an individual detector. Individual micro-lenses are commonly integrated into a separate component, a micro-lens array (MLA), immediately adjacent to the array of display emitters or FPA detectors.
Focusing of light occurs when the optical pathlength (OPL) of light rays passing through a lens varies laterally over the cross section of the lens. Under these conditions, the phasefront of a collimated incident light beam experiences a variable optical path difference (OPD). Conventional lenses are typically made of optically homogeneous glass such that the refractive index is constant inside the lens. The OPL variations are controlled by the geometry of the lens and depend on the lateral coordinates which correspond with variations in glass thickness. A positive lens is thicker near the lens center where lens thickness may, for example, be approximated as a parabolic function, yielding a parabolic shape for the OPD as well. Because of the sharp radius of curvature often desired in lenses for FPA detectors (less than about 1 cm), it is difficult to fabricate a conventional lens with a focus sharper than 100 diopters.
Micro-spheres and optical fibers can be used for focusing light to tiny light spots. Geometry of the shaped glass surface is responsible for focusing. Glass spheres, tens of micrometers in diameter, may provide the focusing strength desired in some FPAs. However, it is very difficult to fabricate a large number of spheres of a calibrated size, and to dispose them as a regular array. Micro-sphere arrays also generally exhibit a high insertion loss due to reflection near the boundaries between neighboring spheres due to the large incidence angles.
Fresnel lenses are also often used for micro optics, being much thinner and lighter as compared to regular lenses. They are made as concentric rings corresponding to Fresnel zones. The zones are separated by abrupt surface steps. Within each Fresnel ring, the surface shape is equivalent to the shape of conventional lens with the same focal distance, if made of the same material. Unconstrained, this design produces a truncated phasefront, instead of a smooth phasefront, resulting in diffractive losses. If the step discontinuities are an integral multiple of the wavelength, the resultant phase is smooth and the diffractive loss due to the transitions is negligible. The tolerances for manufacturing a Fresnel micro-lens are generally very tight. The depth of the surface steps may be on the order of a single wavelength, for example, less than a micron for optical lenses, while the curved surface between the steps must be figured with high precision. Diamond turning and digital control techniques are usually used for manufacturing Fresnel lenses. If the Fresnel lens is cut on a surface of a high-index semiconductor, which is typical material for FPA wafers, an impedance-matching, anti reflection coating applied to the lens surface is often used to reduce insertion loss. The effectiveness of typical antireflection coatings is a strong function of the angle of incidence, which may be quite large for low f/number imaging systems where these MLAs are used.
A Fresnel lens is normally round. Truncating a micro-lens element to fit a rectangular or hexagonal array unit cell shape results in sharp and irregular surface features that are difficult to fabricate for either a segmented or monolithic structure. For FPA applications, a spot size reduction of approximately a factor of two is often desired. Such focusing strength may only require a first Fresnel zone. Therefore, multi-zone Fresnel configurations may add unnecessary complexity relative to a conventional lens shape for FPA sensor applications.
An alternative to a shaped-surface lens is a gradient index (GRIN) lens. GRIN lenses are traditionally fabricated by bonding or fusing a number of glass rods together along their cylindrical surfaces in a regular array pattern, for example, rectangular or hexagonal. Each glass rod may be a few millimeters in length. The optical focusing power derives from using rods with different refractive indices in the array such that the refractive index of the assembly varies in a substantially smooth fashion as a function of radius from the center of the array. For positive focusing power, the refractive index is made denser near the lens center and drops down toward the periphery. After propagating a short distance, along the rod axis, the optical pathlength for different points of the cross section varies with the shape for the OPD, similar to a conventional lens. Index gradients are created by variations of glass composition from the rod center to its periphery, and the focusing power is limited by the maximum variation in refractive index that can be achieved with the glass chemical. Such lenses can be made approaching the thousand diopters level often desired for FPA applications, so that a collimated input beam will focus at the output surface of a few millimeters long rod. Arrays made of such GRIN rods or GRIN fibers, however, may be prohibitively bulky for an FPA micro-lens array application and require costly optical assembly and alignment processes.
Pixel separation (or pitch) for a typical FPA sensor depends on the wavelength of the radiation desired to be used for imaging. For long-wave and mid-wave infrared (IR) spectral bands, typical pixel pitches may be tens of micrometers. For visible and near-infrared spectral bands, pixel pitches may range down to micrometers. The Fresnel range zFr is defined for a collimated light beam as the propagation distance over which diffraction results in an increase in the spot size diameter, such that the light intensity at the center is reduced by a factor of 2. The Fresnel range is of the order of hundreds of micrometers for such pixel dimensions. To focus incident light, the focal distance, F, of a lens should be significantly shorter than the Fresnel range. The Fresnel range also determines the focusing power for individual lens elements in a FPA micro-lens array, according to 1/F>1/zFr. The focusing power should therefore be strong, on the order of thousands of diopters. Standard commercial lenses are generally too bulky and have insufficient optical power for FPA applications as described herein. Accordingly, a micro-lens array is typically desirable for FPA applications. While it is possible to conceive of a micro-lens array as an assembly of individual lens elements, such small diameter and sharp focusing lens components are difficult to make and handle, and integrating these into an array structure requires very difficult fixturing and bonding processes. Consequently, micro-lens arrays are typically manufactured as monolithic arrays.
Conventional micro-lens arrays (MLAs) are usually made as a thin plate of transparent amorphous material, such as glass, silicon, borosilicate glass, plastic, or crystal. One or both of the opposite surfaces is shaped, so the surface profile resembles like an array of bulges, wherein each bulge functions as an individual lens, as shown in FIG. 1. Surface profiling is typically performed using lithography tools, or by stamping, molding, casting, or rolling. Commercial MLA plates are typically relatively thick, thereby increasing the size and weight of usually compact FPA designs. The MLA is a separate component of an FPA sensor system that increases parts count. In addition, the MLA typically requires enhanced manufacturing tolerances, and a special alignment process is needed for FPA sensor system assembly. Furthermore, round-shaped lenses do not fit rectangular or hexagonal pixel cells, and multiple reflections between profiled surfaces give cross-coupling between individual lenses in the array.
Hence there is a need in the art for alternative methods of light concentration and other types of micro-lenses for FPA applications.